It is state of the art practice to extend a gas turbine plant with a waste-heat boiler and to combine the gas turbine plant with a steam turbine. The gas turbine and the steam turbine each drive their own generator or drive a single generator via a common shaft. These combination plants, referred to as combined cycle plants, are generally distinguished by their very good conversion efficiencies which range in the order of magnitude from 50 to 52%. These high efficiencies result from the cooperation of a gas turbine with at least one steam turbine circuit. The gas turbine exhaust gases are passed through the waste-heat boiler and the residual heat potential of these waste gases is utilized for producing the steam required for feeding the steam turbine. LNG has been used in combined cycle plants as a combustion energy source.
LNG is normally transported overseas as a cryogenic liquid in specialized vessels. At the receiving terminal this cryogenic liquid, which is approximately at atmospheric pressure and at a temperature of around -160.degree. C., has to be regasified and fed to a distribution system at ambient temperature and at a suitably elevated pressure, typically ranging up to 80 atmospheres. The liquid is pumped to the required pressure so that when heat is added and it is regasified, no compression of the resultant natural gas is required.
Although many suggestions have been made and some installations have been built to utilize the large cold potential of LNG, in most receiving terminals the cold potential is wasted and the LNG is simply heated with a large flow of sea water which has to be applied in such a manner as to avoid ice formation.
For example, Mandrin et al., in U.S. Pat. No. 3,978,663, disclose a method for improving the efficiency of gas turbines by cooling the intake air with a liquid coolants. Air entering the intake portion to the turbine is filtered and chilled by cooling exchangers. Coolants such as freon, carry heat from the air to vaporize LNG through the exchangers. Methanol is introduced to the air by a mixing means as a deicer to prevent ice blockage in the exchanger, and is separated in a collecting means. The exhaust heat is used to vaporize water separated from the methanol and water mixture. In a subsequent patent Mandrin, in U.S. Pat. No. 4,036,028, discloses the use of multiple work liquids in conjunction with an open gas turbine system. The super cold from LNG is used to extract heat from the intake air by means of heat exchangers and a freon carrier fluid. An antifreeze liquid, such as methanol, is injected to prevent icing in the exchanger. The reference discloses a vapor turbine positioned in-line with a compressor.
A power generation system is also disclosed by Kooy et al., in U.S. Pat. No. 4,995,234. This patent discloses the chilling of intake air for a turbine, the warming of LNG by passing it against condensed carbon dioxide, and the use of a gas turbine exhaust system for heating a material used to drive a gas turbine.
Similarly, Nozawa, in U.S. Pat. Nos. 4,330,998, 4,429,536, and 4,422,298, discloses a liquefied natural gas-refrigerant electricity generating system. Generally, these patents teach a supply of freon being compressed and heated. The compressed/heated freon is then used to drive a high pressure turbine. The freon is heated again and passed through a low pressure turbine before being cooled in a heat exchanger against a nitrogen and/or LNG stream.
Combined gas and steam power plants are disclosed by Woolley, in U.S. Pat. No. 3,605,405, and Keller et al., in U.S. Pat. No. 4,953,479. The Keller reference particularly discloses methacoal integrated combined cycle power plants using a gas turbine and steam turbine system where the exhaust from the gas turbine system is used to produce steam to drive the steam turbine. The used steam is then condensed and reheated by the exhaust system.
None of the foregoing, or other power generation systems utilizing LNG, address the problem of maximizing the efficiency and capacity of a gas powered turbine. More particularly, none of the references address maximizing the efficiency and capacity of a gas powered turbine in warm climates, wherein the peak electrical consumption occurs when the air temperature is at its highest. Typically, the efficiency and capacity of gas turbine decreases with increasing air temperature.
It is therefore an object of this invention to provide a cogeneration system for generating electricity and gaseous hydrocarbon from a liquefied cryogenic material such as LNG.
A further object of this invention is to provide a cogeneration system which utilizes a liquefied cryogenic material, such as LNG, to densify (by reducing the air temperature) the intake air to maximize the efficiency and capacity of a gas turbine in warm climates.
The invention broadly embodies a system and process which improves the capacity of a combined cycle plant in an amount up to 9% and the efficiency of the plant up to about 2%, particularly when the ambient temperature exceeds 60.degree. F. A LNG fuel supply system is used in combination with a combined cycle plant. The combined cycle plant comprises a gas turbine plant, a waste-heat boiler and a steam turbine plant. A primary heat exchange fluid is chilled in the LNG fuel supply system and is then utilized in the gas turbine process to cool and densify the intake air to the gas turbine. The primary heat exchange fluid is also utilized in the steam turbine process to condense the spent steam from the steam turbine. Lastly, the primary heat exchange fluid is recycled to the LNG fuel supply system where it is rechilled. The primary heat exchange fluid flows through a closed loop while cooling and densifying the intake air, while condensing the steam discharged from the steam turbine and when being rechilled in the LNG fuel supply system.
The LNG fuel supply system comprises the LNG supply, a regasifier and a chiller. In the LNG fuel supply system is a secondary heat exchange fluid which flows through a closed loop. The secondary heat exchange fluid is in heat exchange relationship with both the regasifier where the LNG is converted to natural gas, and the chiller where the primary heat exchange fluid is chilled. The natural gas is used, in part, as the fuel for the combustor in the gas turbine plant. The secondary heat exchange fluid is cooled in the gasifier, by the expanding LNG, and chills the primary heat exchange fluid in the chiller. The LNG is regasified without the use of expensive sea water regasifiers and/or without the need for using fuel for the heat source.
In a preferred embodiment of the invention, the primary heat exchange fluid, water, flows through the water chiller (heat exchanger) in the LNG fuel supply system. The secondary heat exchange fluid, a water/glycol mixture, chills the primary heat exchange fluid which primary heat exchange fluid then flows to a heat exchanger in the gas turbine plant. The gas turbine plant, which is fueled by the regasified LNG, drives a generator. The gas turbine plant has an air intake duct, a heat exchanger, a water separator, an air compressor, a combustor, a gas turbine and an exhaust port. The heat exchanger is positioned within the air intake duct. The primary heat exchange fluid flows through the heat exchanger and supplies a chilled refrigerant stream for densification and cooling of the air intake stream to the air compressor.
A waste-heat boiler is downstream of and in communication with the exhaust port of the gas turbine. The exhaust of the gas turbine converts a stream of water flowing through the boiler into high pressure steam.
The steam turbine plant comprises a steam turbine and a condenser for spent steam. The high pressure steam from the boiler is used to drive the steam turbine. The spent steam from the turbine flows into a condenser. The primary heat exchange fluid flows through the condenser and condenses the spent steam. The primary heat exchange fluid then returns and flows through the chiller in the LNG fuel supply system.